The block may be a Block object that contains transactions, a Block object that is only a header when
fast catchup is being used. If set, filteredBlock can be used to retrieve the list of associated transactions.

onPreMessageReceived

Called when a message is received by a peer, before the message is processed. The returned message is
processed instead. Returning null will cause the message to be ignored by the Peer returning the same message
object allows you to see the messages received but not change them. The result from one event listeners
callback is passed as "m" to the next, forming a chain.

getData

Called when a peer receives a getdata message, usually in response to an "inv" being broadcast. Return as many
items as possible which appear in the GetDataMessage, or null if you're not interested in responding.

onPeerDisconnected

Called when a peer is disconnected. Note that this won't be called if the listener is registered on a
PeerGroup and the group is in the process of shutting down. If this listener is registered to a
Peer instead of a PeerGroup, peerCount will always be 0. This handler can be called without
a corresponding invocation of onPeerConnected if the initial connection is never successful.